1. Field of the Invention
The present invention relates to a polarization beam splitting element that splits an entering beam according to polarization directions, and particularly to a polarization beam splitting element that is constituted by a wire grid (one-dimensional grating structure) having a grating period smaller than a wavelength of the entering beam.
2. Description of the Related Art
A wire grid that is a one-dimensional grating structure formed of a metal and having a grating period smaller than a wavelength (also referred to as a “use wavelength”) of an entering beam has a function of splitting beams whose polarization directions are mutually different. Japanese Patent Laid-Open No. 2003-502708 discloses a wire grid having a visible wavelength-order minute grating period and formed on a substrate.
Such a wire grid reflects polarized light (first polarized light) whose electric field oscillates in a plane including a grid longitudinal direction and transmits light (second polarized light) whose electric field oscillates in a direction perpendicular to that plane.
Moreover, Japanese Patent Laid-Open No. 2003-519818 discloses a wire grid polarization beam splitting element that is used by being sandwiched between transparent substrates such as prisms, for the purpose of suppressing generation of astigmatism at the substrate and of improving handling easiness of grating.
However, the wire grid requires increase of a grating thickness thereof in order to achieve a sufficient polarization splitting function, which causes the wire grid to absorb the second polarized light to be transmitted and thereby reduces transmittance for the second polarized light.
Furthermore, the wire grid polarization beam splitting element sandwiched between the transparent substrates such as the prisms tends to have low optical performance for the second polarized light, that is, has a higher reflectance for the second polarized light as compared with a case where an air space or a vacuum space is formed on a beam entrance side of the wire grid. Thus, using such a wire grid polarization beam splitting element for color separation in an image projection apparatus makes it impossible to use a same optical path for respective colors, which increases cost of the apparatus and makes it difficult to miniaturize the apparatus.